All species have to face challenges whilst in the river, whether they permanently reside in the freshwater environment or utilise it as part of their life cycle. Notwithstanding the inevitable and considerable pressures on fish populations from human interference in the riverine ecosystem (for example, from fishing, pollution, water abstraction) there are also dangers from disease, parasites and predators, which can have devastating effects on fish, whether species-specific, geographically isolated, or on a particular stock as a whole.